1. Field of the Invention
The present invention relates to methods and apparatus for stabilization of scale factor in a fiber optic gyroscope (xe2x80x9cFOGxe2x80x9d). More particularly, the invention is directed to fiber optic gyroscopes equipped with a broadband light source, such as a superluminescent diode (xe2x80x9cSLDxe2x80x9d) or a multimode laser.
2. Description of the Prior Art
Broadband SLDs are often employed in relatively new fiber optic gyroscopes without bandwidth limitation. It has been found, in existing SLDs, that aging effects and instantaneous operating temperature fluctuations lead to significant scale factor fluctuations, unacceptable for high precision gyros.
Past attempts have been made to counter the fluctuation problem by measuring temperature at the FOG sensor block and the light source to correct wavelength-dependent error. Relatively-complex models have led to reduction of the wavelength uncertainty, but fail to provide active compensation. Instantaneously detected temperature-dependent wavelength fluctuation magnitudes are on the order of 300 nm/degrees C. magnitude. The aging effect depends primarily upon production processes.
GB 2 157 425 A teaches scale factor stabilization in a fiber optic gyroscope incorporating a temperature compensation apparatus for adjusting the laser radiation wavelength to minimize the temperature coefficient of scale factor. As a result, the data emitted from the gyro are not influenced by changes in the temperature of the fiber.
JP 08304084 A teaches an electronic signal processing circuit for correcting the scale factor error of an optical gyroscope. To accomplish this, wavelength change is detected by measuring polarization level change. The polarization level of the light passing through the birefringent optical fiber varies as the wavelength of the light source varies in response to temperature changes.
DE 34 29 592 A1 discloses a fiber optic gyroscope including a temperature compensation device. The arrangement insures that the wound fiber has a thermal coefficient of expansion having a value approximately equal to the refractive-index coefficient of the fiber. As a result, the scaling-factor temperature coefficient is reduced. The temperature compensation apparatus may be in the form of a molded body element, or may be applied to the fiber as a coating.
CA-A-1,289,787 teaches a proposal for improving scale factor accuracy of an FOG in which an optical bandpass filter centered on the main wavelength of the light source is positioned within the light path upstream of the detector. The filter has a bandwidth that is narrower than the spectrum of the light beam emitted from the light source (SLD).
It is therefore the object of the present invention to provide a relatively-simple apparatus and method for improving scale factor stability in a fiber optic gyroscope.
The present invention addresses the preceding and other objects by providing, in a first aspect, a method for scale factor stabilization of a fiber optic gyroscope of the type in which the emission bandwidth of the intensity characteristic of a light source injected into a fiber end is limited by a narrowband optical filter to a considerably narrower transmission bandwidth. Such method comprises the step of readjusting the transmission bandwidth of the optical filter as a function of the measured temperature at the filter.
In another aspect, the invention provides an improvement in a fiber optic gyroscope of the type that includes an optical filter arrangement in the light path for limiting the bandwidth of the light carried in the fiber. The passband characteristic of the optical filter arrangement is matched to the wavelength band of the gyroscope. The improvement provided by the invention includes a temperature measurement device for measuring the instantaneous temperature of the optical filter arrangement and a device for correcting the measured temperature value. Such device employs a calibrated, stored temperature wavelength table that supplies a corrected value for calculating the scale factor via the gyroscope electronics.
The preceding and other features of the invention will become further apparent from the detailed description that follows. Such description is accompanied by a set of drawing figures. Numerals of the drawing figures, corresponding to those of the written description, point to the various features of the invention with like numerals referring to like features throughout the written description and the drawing figures.